Uninterruptable power supplies (UPS) are widely applied for a variety of critical loads, including computers, financial transaction handlers and life support equipment. A proliferation of power electronic loads on the utility grid has caused a significant increase in harmonic levels on the AC lines. Further, the use of computers on a very large scale has made the issue of the cleaness of the power available on the power system more critical. Although UPS systems have been used in the past for large computer installations, the increased dependency of business and industry on computers has seen a tremendous increase in the use of UPS systems even for small computers operating off single phase supply lines.
Given the decreasing cost of computing capability, single phase UPS systems are extremely sensitive to cost. Consequently, few UPS manufacturers can afford the luxury of offering improved power factor interface to the utility. However, the possibility of harmonic standards being enforced is very real, and more importantly, the restriction of RMS current supplying capability for commercial or domestic line frequently limits the rating of the largest unit that can be fed from the available supply. Consequently, units with a poor power factor will be able to draw less power from the supply mains than a comparable unit with sinusoidal input line currents. A third harmonic overload in neutral conductors is another result of AC line current harmonics, and are often encountered in the use of typical UPS systems and switched mode power supplies.
Two approaches have primarily been utilized for commercial UPS systems. One approach uses a ferro-resonant transformer as a voltage regulator. Line conditioning is completely passive and the technique is very robust, giving good immunity against disturbances. Under normal operation therefore, power flow does not occur through the transfer devices. On identifying failure of the AC line, the inverter is brought on line, the connection to line power is cut off, and the inverter continues supplying the load. During normal operation, the inverter can also be used to recharge the battery. This technique has high efficiency and reliability as well as moderate cost. However, the ferro-resonant transformer tends to be fairly heavy, and transitions from AC line to inverter operation can encounter problems under certain low or high line conditions. The quality of the output waveform under non-linear loads can also be very poor.
The other common approach is the so-called on-line UPS system in which a separate AC to DC converter is used to provide battery charging and regulation function. The DC bus is supported by the battery and feeds a DC to AC inverter with appropriate filtering. Static by-pass switches are used to revert the output power to the AC line in the event of failure of the inverter. Such a scheme provides power from the main power lines under normal operation, but only after two stages of power processing. Consequently, although the system has good performance, it is more expensive and less efficient.
Another approach reported recently may be referred to as a stand-by utility interactive UPS system. See, T. Kawabata, et al., "Chargerless UPS Using Multi-Functional Mi-MOS Inverter," IEEE-IAS Conf. Rec., 1986, pp. 513-520. Using such a technique, the need for two power conversion stages is eliminated, and the inverter charges a high voltage DC battery while the system is in the stand-by mode. This approach, however, allows no line conditioning in terms of voltage correction, disturbance rejection, or reactive/harmonic compensation. The approach can be varied so that the inverter is used to simultaneously function as an active filter to improve the line current waveform, as well as to maintain the state of the charge on the battery. This approach requires higher inverter ratings and bulky series filter components.
High frequency link systems which would result in substantial reductions in transformer size and cost have been proposed. See, e.g., S. Manias, et al., "Bilateral DC to AC Converter Employing a High Frequency Link," IEEE-IAS Conf. Rec., 1985, pp. 1156-1162; I. Yamato, et al., "New Conversion System For UPS Using High Frequency Link," IEEE-PESC Conf. Rec., 1988, pp. 658-663. While the concept of high frequency links is very sound, its usefulness in UPS applications may be questionable. Upon inverter failure, the bypass arrangement now has to be accomplished without isolation. If the system is to operate under those conditions for any length of time, it may not be acceptable. Further, the presence of a static bypass arrangement already involves non-isolated contact between the supply and load sides, completely obviating the function of the high frequency transformer. However, the objectives of UPS availability can also be provided for by multiple units operating in parallel.
None of the approaches discussed above simultaneously satisfy the requirements of load regulation with nonlinear loads, single stage power conversion, small filter components, and sinusoidal AC line currents with unity power factor.